There is a need for high quality electron beams in a number of applications, including modern energy recovery linac (ERL) based applications, and high power, high efficiency RF (radio frequency) amplifiers. The need for high peak, high average current, and high quality electron beams is particularly strongly in facilities that perform scientific research in nuclear physics, for example the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL), and the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (JLab).
Typically, thermionic electron RF guns include a thermionic cathode, which is heated to create a stream of electrons via thermionic emission. Thermionic cathodes are capable of generating high quality beams with high peak and high average current, in a variety of vacuum electronic applications. However, thermionic cathodes cannot generate such beams in RF cavities. They have thus been replaced by photocathodes for most RF gun applications that require high-brightness electron beams. Not only are photocathode sources very expensive, but also it is very hard to obtain high average current with them. Further, the lifetimes of photocathodes are also very short.
A major reason that prevents a thermionic cathode from generating high-brightness, high average current beam in RF guns is the un-controlled phase of the emission, which spans from 0° to 180° of an RF cycle, and which causes back-bombardment.